1. Field of the Invention
The present invention relates to a grease composition and more particularly, to the grease composition comprising a silicon oil of 61 to 47% by weight as a base oil, a polytetrafluoroethylene(PTFE) of 35 to 40% by weight as a thickener, a silica aerogel of 3 to 8% by weight and an extreme pressure additive of 1 to 5% by weight which provides improved abrasion resistance, heat resistance, cold resistance, low torque property at low temperature, and viscosity, to be applied in the fields of automobiles and aerospace.
2. Description of Prior Art
Grease is a semisolid type lubricant manufactured by mixing of a thickener with a base oil. Grease is mainly used for lubricating in an engine exposed to heavy load or high temperature and in places for high speed. A diester oil and silicon oil are widely used as a base oil of grease composition having a fluidity at low temperature, and a metallic soap, a silica aerogel and carbon black are used as a thickener.
Silicon oil which is mainly used as the base oil of a grease composition has low changes of an coefficient of viscosity over a wide ranges of temperatures, constant fluidity at low temperature since a solidification point is low. Further, silicon oil is chemically inactive and has low surface tension, water resistance, a defoamance, a releasing action and high electrical insulating property and thus it can be extended its applicability to a variety of fields.
Also, a thickener added to a grease composition gives a base oil a non-Newton property to be maintained in a semisolid phase. Conventionally, a metallic soap, ureas, a silica aerogel, a bentonite and carbon black are used as a thickener. Particularly, the metal soap is a metal salt of a fatty acid which is one of main materials of the soap thickener. Main materials of the soap thickener are divided into a fatty acid and a metal hydroxide. A fatty acid used mainly is a refined stearic acid, and a metal source is a hydrate of an alkali metal such as lithium or sodium or an alkali earth metal such as calcium or barium. However, since a metal component of a thickener may act as a catalyst which causes to oxidize and corrode, the use of such a thickener has drawbacks that an oxidation stability and heat stability of a grease composition become lowered, it is difficult to use for a heavy load condition and abrasion resistance becomes poor.
In the mean time, diurea, triurea, tetraurea and polyurea are also used as a urea thickener. Compared with the metal soap, these urea thickeners have a superior heat stability, oxidation stability, abrasion characteristic and resistance to water but they have poor inferior affinity with silicon oil.
According to the choice of a thickener, physical and chemical properties of a grease composition are varied, therefore, it is very important to choose an appropriate thickener.
Specially, a grease composition has been used for lubricating in various ultramodern machineries, automobiles and aerospace industries, however, a grease containing a silicon oil as a base oil has lower abrasion resistance and extreme pressure resistance so that the grease is restricted to use in a high load lubricant condition between metals. In order to overcome this limitation, a fluorine containing silicon oil that displaces a methyl group of a backbone of the silicon to fluorine is developed. However, the fluorine containing silicon oil is much more expensive than a dimethyl silicon oil or diphenyl silicon oil which is conventionally used. Therefore, a usage of the additive or usage of a special thickener must be considered in order to overcome an extreme pressure problem.